Certain spectroscopy techniques feature the enhancement of a spectroscopic signal through electromagnetic interaction at a surface. Representative surface enhanced spectroscopic (SES) techniques include, but are not limited to surface enhanced Raman spectroscopy (SERS) and surface enhanced resonance Raman spectroscopy (SERRS). In SERS or SERRS, a metal or other enhancing surface will couple electromagnetically to incident electromagnetic radiation and create a locally amplified electromagnetic field that leads to 102- to 109-fold or greater increases in the Raman scattering of a SERS active molecule situated on or near the enhancing surface. The output in a SERS experiment is the fingerprint-like Raman spectrum of the SERS active molecule.
SERS and other SES techniques can be implemented with particles such as nanoparticles. For example, gold is a SERS enhancing surface, and gold colloid may be suspended in a mixture to provide for enhanced Raman spectrum detection. SERS may also be performed with more complex SERS-active nanoparticles, for example SERS nanotags, as described in U.S. Pat. Nos. 6,514,767, 6,861,263, 7,443,489 and elsewhere. In a SERS nanotag, a reporter molecule is adsorbed to a SERS-active surface, and both the SERS-active surface and the reporter are encapsulated, typically with silica or another relatively impervious material. One advantage of a silica or glass coating is that it prevents the adsorbed molecule from diffusing away. The coating or shell also prevents other molecules from adsorbing to the enhancing surface or particle core. This configuration imparts a level of robustness and environmental insensitivity to the particles that is, for many applications, a desirable feature.
Environmental insensitivity and robustness will cause a SERS nanotag to be spectroscopically static. In many implementations, it is desirable that a SERS nanotag returns the same signal virtually no matter how long the tag has been applied to an item or embedded in a substance and no matter how many types of compound or solution are contacted with the SERS nanotag. It is also desirable, but problematic, that a SERS nanotag or similar taggant be relatively insensitive to temperature fluctuations. In particular, it is desirable that the signal capacity of taggants used to mark substances or items that are subjected to elevated temperatures not degrade as a function of elevated temperature. This is problematic in the case of SERS nanotags as described in U.S. Pat. Nos. 6,514,767, 6,861,263, 7,443,489 and elsewhere because the organic reporter molecules described therein can degrade and lose SERS activity at certain elevated temperatures.
For example, graph 100 of FIG. 1 shows the spectroscopic intensity of SERS nanotags with a selected reporter molecule upon interrogation at room temperature. Graph 102 shows the spectroscopic intensity obtained from the same SERS nanotags, after the tags were held for 8 minutes at 250° C. in an environmentally sealed thermal stage. It is clear from a comparison of graph 100 with graph 102 that elevated temperature caused substantial degradation of the signal that can be obtained from this SERS nanotag and reporter combination.
The present invention is directed toward overcoming one or more of the problems discussed above.